Chatham Grammar School for Boys in Kent was declared inadequate after a visit by Ofsted in June and placed in “special measures”.

In its report, inspectors criticised standards of teaching, leadership, behaviour in lessons and pupil achievement.

The move comes despite the fact that 95 per cent of pupils gained five good GCSEs including English and maths this summer.

A new team of education experts is now being brought in to manage the school, which is run free from local authority control as an academy. It will be subjected to regular inspections by Ofsted until major improvements are made.

Chatham is only the second academically-selective grammar school to be placed in Ofsted’s lowest-possible category.

In 2009, Stretford Grammar in Manchester was placed in special measures after the watchdog criticised standards of leadership at the school. Ofsted’s latest report in 2012 branded it a “good and rapidly improving school”.

The latest conclusion comes after the introduction of a tough new Ofsted inspection regime that challenges schools to show that even the brightest children are making good progress.

It comes after Sir Michael Wilshaw, the chief inspector, warned that too many gifted pupils were being allowed to languish in mixed-ability lessons.

Denise Shepherd, executive principal of nearby Rochester Grammar School Trust, has now become interim executive principal of Chatham Grammar.

Rochester Grammar's principal, Stuart Gardner, has been appointed interim principal.

Ms Shepherd said insisted that the school was making progress but Ofsted had “challenged the rate of improvement”.

"The new interim principal and I will be working with all leaders and teachers across the academy to bring about the necessary change,” she said. “As part of that we will be regularly monitoring lessons and teacher involvement across the curriculum."

In its report, Ofsted said the leadership and management of the school was substandard and teaching had not improved over the last few years.

It also warned that achievement in key subjects – particularly English – was poor and pupils' achievement was not consistently good enough.

The report also concluded that pupils' behaviour in lessons had worsened, warning that many children "lose interest and motivation when teaching is not good enough".

Inspectors reported the "teaching requires improvement because weak and inadequate teaching has not been eradicated".

However, the report said that pupil behaviour around the school was good and students with special educational needs made decent progress.